The present embodiments relate to a method for checking a high-frequency transmit device of a magnetic resonance tomography system.
In a magnetic resonance system, a body to be examined may be exposed to a relatively high basic field magnetic field of, for example, 3 or 7 tesla with the aid of a basic field magnet system. A magnetic field gradient is also applied with the aid of the gradient system. High-frequency excitation signals (HF signals) are emitted by a high-frequency transmit system using suitable antenna devices. The high-frequency excitation signals are used to tip the nuclear spin of certain atoms that have been excited in a resonant manner by this high-frequency field, with spatial resolution through a defined flip angle relative to the magnetic field lines of the basic magnetic field. During relaxation of the nuclear spin, high-frequency signals (e.g., magnetic resonance signals) are emitted. The high-frequency signals are received using suitable receive antennas and are further processed. Raw data thus acquired may be used to reconstruct desired image data. The emission of the high-frequency signals for nuclear spin magnetization may take place using a “whole-body coil” or “body coil,” or also (in the case of many measurements) using local coils that are placed on the patient or participant. A typical structure of a whole-body coil is a birdcage antenna including a plurality of transmit rods that are disposed in parallel with the longitudinal axis around a patient chamber of the tomography system, in which a patient is present during the examination. End faces of the antenna rods are respectively connected in a capacitive manner in a ring.
The whole-body antennas may be operated as “volume coils,” in, for example, a “CP mode,” in which a circularly polarized high-frequency signal (HF signal) is transmitted as homogeneously as possible into the whole of the volume that is enclosed by the whole-body antenna. For this purpose, a single temporal HF signal is emitted to all components of the transmit antenna (e.g., all transmit rods of a birdcage antenna). In this case, the pulses may be transferred to the individual components in a phase-shifted manner with a displacement that is adapted to the geometry of the transmit coil. For example, in the case of a birdcage antenna with 16 rods, the rods may each be shifted through 22.5° phase displacement with the same HF magnitude signal.
Such a homogeneous excitation results in a global high-frequency exposure (HF exposure) of the patient. The global high-frequency exposure is to be limited according to the usual rules, as too high a high-frequency exposure may harm the patient. HF exposure may include not only the HF energy that has been introduced, but also a physiological exposure that has been induced by the HF irradiation. A typical measure of the high-frequency exposure is the specific absorption rate (SAR) value that indicates, in watts/kg, the biological exposure acting on the patient due to a certain high-frequency pulse output, or the specific energy dose (SED). Either value may be converted into the other. According to the IEC standard, a standard limit of 4 watts/kg at a “first level” currently applies with respect to the global whole-body SAR or HF exposure of a patient, for example. Accordingly, the total power that is absorbed by a patient must not exceed a value of 4 watts/kg in a time window that is averaged over six minutes. In order to provide this, the high-frequency exposure is monitored continuously during each measurement by suitable safety devices on the magnetic resonance system. A measurement is changed or terminated if the SAR value exceeds the specified standards.
In the context of more recent magnetic resonance systems, individual HF signals tailored for imaging purposes may be assigned to the individual transmit channels (e.g., the individual rods of a birdcage antenna). A multichannel pulse train including a plurality of individual high-frequency pulse trains that may be emitted in parallel via the different independent high-frequency transmit channels is emitted for this purpose. Such a multichannel pulse train (e.g., a “pTX pulse” due to the parallel emission of the individual pulses) may be used as an excitation, refocusing and/or inversion pulse, for example. Such a transmit antenna system, which allows the parallel emission of pTX pulses via a plurality of independent transmit channels, is also referred to as a “transmit array,” which term is used in the following (irrespective of the appearance in detail of the architecture of the transmit antenna system).
During the emission of multichannel pulse trains, the previously homogeneous excitation may be replaced by an excitation of any form in the measurement chamber and hence also in the patient. In order to estimate the maximal high-frequency exposure, every possible high-frequency superimposition is therefore to be considered.
The monitoring of the superimposed electrical fields over a plurality of individually activatable antenna elements is important, for example, because the electrical fields add up in a vectorially linear manner. The local power release and hence the exposure of the patient at the relevant location is proportional to the square of the resulting electrical field.
The local high-frequency exposure may not be measured directly. Suitable body models having a (complex) conductivity distribution are therefore created, and the suitable body models are used to calculate the fields that are produced by the respective antenna elements at the individual locations of the model. Such calculations are performed in the prior art using a finite different time domain (FDTD) method, for example. In this case, the examination object may be divided into a plurality of voxels, and the electrical field strengths that are produced by the individual antenna elements, and the superimposition thereof, are determined for each voxel. Given the plurality of voxels concerned (e.g., many models have 50,000 voxels, others have many more than 100,000 and even several million voxels in extreme cases) and due to the complexity of the calculations to be performed, realtime monitoring or online capability may not be provided when such an approach is used.
DE 10 2009 030 721 describes a method and a device for SAR monitoring, in which cross-correlation matrices of excitation vectors relating to the individual antenna elements are determined for a plurality of time points or time periods in each case. The cross-correlation matrices are added up over a summation time period. The sum matrix is multiplied by a number of hotspot sensitivity matrices. Each hotspot sensitivity matrix represents the sensitivities at a plurality of points of the examination object with reference to the calculation of the maximal local high-frequency exposure (e.g., SAR). Such “hotspots” may develop in the high-frequency field in the patient. The applied HF power and hence the physiological high-frequency exposure at the hotspots may produce a multiple of the values previously experienced from the homogeneous excitation. In order to provide that the local high-frequency exposure does not exceed the limit values, it is therefore only necessary to monitor these hotspots. DE 10 2010 011 160 also describes a method that facilitates correct selection of the hotspots with reference to previously performed electromagnetic simulations (e.g., FDTD).
The flexibility in the excitation of spin magnetizations using transmit arrays allows emission of a homogeneous high-frequency signal in the whole volume, as per the previous volume coils. A transmit array may also be operated in a “volume coil mode,” which may be appropriate for many measurements and/or examinations. Such a “volume coil mode” or “homogeneous mode” in the following may be an operating mode of the high-frequency transmit device, in which the transmit array is operated in a manner equivalent to that of a volume coil (e.g., the transmit channels are operated using high-frequency signals that have a fixed amplitude and phase relationship).
Accordingly, the present regulations and/or standards relating to the high-frequency exposure also stipulate that transmit arrays feature properties of both volume coils and local coils, and that the monitoring rules to be applied in each case (e.g., the limit values) depend on the way in which the transmit array is used. In this context, monitoring of the local SAR is required when used in the manner of local coils (e.g., with low limit values that are relatively narrowly defined), while only a whole-body SAR or exposed-part-of-body SAR is monitored when used as a volume coil.
In order to operate a transmit array as a volume coil, a “Butler matrix” may be used between the high-frequency amplifiers of the high-frequency transmit system and the antenna elements. All of the transmitters may be switched off except for the transmit channel that is used by the Butler matrix for the volume mode. The Butler matrix is equivalent to an electrical hardware-type connection of the transmit arrays as a volume coil. However, such a hardware connection has the disadvantage that the already available high-frequency amplifiers of the individual transmit channels are unevenly utilized. While one amplifier of a channel is to deliver the full high-frequency power, the amplifiers of the other channels remain unused. A further consequence is that at least the high-frequency amplifier used for the volume coil mode, and all subsequent components in the transmit channel, are configured for a significantly higher high-frequency voltage. This makes the system relatively expensive.
The transmit array may be operated in a volume coil mode by using software alone to activate the high-frequency amplifiers of the individual transmit channels such that the individual elements of the transmit arrays are operated using relative amplitude and phase relationships that have been preset correspondingly. The operating mode of the transmit array therefore only differs from an operating mode, in which individual non-homogeneous fields are emitted, in that corresponding commands are output to the high-frequency amplifier (e.g., the pTX pulse is selected accordingly). Seen from the transmit side, the transmit array is therefore classified as a use in the form of local coils, and therefore the local SAR limit values (as required for local transmit coils) that are far too low would actually apply (e.g., even though it would be sufficient only to apply the whole-body SAR limit values due to the operation in the volume coil mode). The local SAR limit values, which are far too low for the volume coil mode, would therefore not allow the operation of the customary single-channel volume coil application.